1. Field of the Invention
This invention pertains generally to fixtures and calibration spheres used for the calibration and accuracy verification of coordinate measurement machines (CMMs), and more particularly to integrated fixtures comprising a calibration sphere permanently or removably attached to a stepped calibration fixture reference for improved CMM qualification and calibration.
2. Description of Related Art
Coordinate measurement machines (CMMs) are widely used in industry for dimensional verification of miscellaneous parts. CMMs easily have measurement precision in the 0.0001 inch range (2.54 μm) or better. However, such measurements must be traced to dimensional standards in order to validate their accuracies. This process is known as calibration.
A simplistic non-CMM calibration occurs when one measures a precision one inch steel gage block (here the measurement reference) with a micrometer or vernier caliper, which in turn reads 1.000 inches indicated on the device when in calibration. CMMs are much more complicated, able to render coordinate measurements in three dimensions with much higher precision.
Precision is typically used in the science and engineering communities to describe the repeatability of a set of measurements. Accuracy, however, is the degree in which a measurement reflects a correct value. Typically, engineering measurements are described as a number plus or minus a tolerance. For high precision designs, one may turn to even more complex tolerance annotations, such as ANSI Y14.5.
U.S. Pat. No. 5,430,948, entitled “Coordinate Measuring Machine Certification System”, was issued on Jul. 11, 1995, and is hereby incorporated by reference in its entirety (hereinafter referred to as the '948 patent). The '948 patent provides a method and apparatus for certifying a coordinate measuring machine that includes a certified ballbar, having a pair of spherical surfaces connected with a bar and having certified diameters separated by a certified distance, that is positioned with a positioning device. The positioning device includes a bar support that supports the ballbar at a midpoint of the bar between the balls, wherein the ballbar is supported free of attachment to the balls. The positioning device includes a first rotational assembly for rotatably supporting the ball support for rotation motion about a horizontal axis and a second rotational assembly for rotatably supporting the bar support about a vertical axis. The first rotational assembly provides rotation of the bar support in a clockwise direction to position the ballbar in a first set of angular orientations and in a counterclockwise direction to position the ballbar in a second set of angular orientations. The second rotational assembly provides rotation of the bar support in substantially a complete revolution. While the '948 patent may allow for manual or computer programmed measurement operations, it does not allow for quick calibrations using calibration bars having precisely stepped features at calibrated locations so as to calibrate a CMM over an entire length of distance.
Typical methods of CMM calibration a distance involve a sequence of steps first measuring a stand-alone calibration sphere, removing the calibration sphere, then attaching a stepped calibration fixture along one or two dimensions, and finally measuring the calibration fixture at one or more of the prescribed calibrated surfaces. Should an error occur during the calibration fixture measurement process, the calibration fixture must then be removed, and then the entire calibration sequence repeated with replacement of the calibration sphere. Such iteration step requires on average about 30 minutes for experienced measurement scientists, a significant time in the day of a work shift on an expensive CMM.